The present invention relates to active-matrix substrates for use in matrix-type display devices such as liquid crystal display devices. More particularly, it relates to an active-matrix substrate in which a gate signal line and a source signal line are shortcircuited by means of a shortcircuiting ring to prevent static electricity from causing a break of a signal line or thin film transistor and an insulation breakdown between signal lines, and which can be tested for its characteristics of pixels such as thin film transistor prior to being assembled into a display device.
A typical matrix-type display device such as a liquid crystal display device (hereinafter referred to as "LCD") comprises a pair of opposing substrates sandwiching a display material such as a liquid crystal which is adapted to be selectively applied with a voltage. As shown in the schematic equivalent circuit of FIG. 8, one of these substrates includes source signal lines 5 and gate signal lines 4 which intersect each other in a matrix manner, and, in each region defined by these lines 4 and 5, a pixel electrode 2, a thin film transistor (hereinafter referred to as "TFT") 1 as a switching element for selectively applying a voltage to each pixel electrode 2, and a capacitor 3 for retaining electric charge.
Such an active-matrix substrate is likely to generate static electricity fabrication in various processes and handling because it undergoes frictional treatments such as a rubbing treatment in its fabrication procedure and in the later liquid crystal panel constructing process. Discharge of thus generated static electricity may cause the TFT 1 to be broken or the insulation film at the intersection of signal lines 4 and 5 to be broken down. To avoid a break due to such static electricity, a countermeasure is taken such that respective input terminals 5a, 4a of the source and gate signal lines 5, 4 are shortcircuited to a low electric resistance shortcircuiting ring (hereinafter referred to as "low-resistance shortcircuiting ring") 7 provided in the periphery of the substrate by way of low electric resistance lines 7a formed of the same material as that of the source signal line 5 or gate signal line 4, as shown in FIG. 8.
The active-matrix substrate shown in FIG. 8, as disclosed in, for example, Japanese Unexamined Patent Publication No. 290624/1991, is provided with the low-resistance shortcircuiting ring 7 in its periphery to which respective input terminals 5a, 4a of the source signal line 5 and gate signal line 4 are connected by way of shortcircuiting lines 7a. Accordingly, even if the substrate is charged with static electricity, the static electricity is discharged through the shortcircuiting ring 7 thereby preventing insulation breakdown between signal lines and breakage of a TFT. However, since the source signal line 5 and the gate signal line 4 are connected to each other through a low-resistance line, application of a voltage for testing each pixel for its characteristics of pixels such as TFT or for a possible shortcircuit would cause a heavy electric current to flow. It is, therefore, impossible to check the substrate for an interlayer shortcircuit between gate and source or to measure the characteristics of pixels such as TFT.
For this reason, it is a conventional practice to carry out an electric test on the active-matrix substrate by applying a voltage across respective input terminals 5a, 4a of the source signal line 5 and gate signal line 4 after bonding the active-matrix substrate to its counterpart substrate, introducing a liquid crystal material to between the substrates and cutting off the aforementioned low-resistance shortcircuiting line 7a. Accordingly, when a failure such as insulation failure between signal lines is found out in the test, the entire panel including the two substrates becomes defective. Since parts such as a color filter provided to the counterpart substrate in particular are expensive, failure of such parts becomes one of the causes of increased cost. In addition, since the shortcircuit test must be carried out at the final stage of the fabrication of the active-matrix display device, possible process abnormalities during the substrate fabrication cannot be fed back promptly, thus interfering with improvements in production yield and product quality.
Japanese Unexamined Patent Publication No. 27263/1993 has proposed a method of forming a TFT of the same type as the TFT forming part of each pixel between shortcircuiting ring 7 and each signal line to insert a resistor therebetween. However, the resistance of the resistor is extremely high, or several mega-ohms (M.OMEGA.) and, hence, this method is inadequate as a countermeasure against the problem of static electricity.
Japanese Unexamined Patent Publication No. 116117/1991 has disclosed a device wherein a shortcircuiting ring is formed of an MoTa or ITO film, the material of which is the same as that of a gate signal line or pixel electrode, to have a high electric resistance (hereinafter referred to as "high resistance") in a range required for the countermeasure against the problem of static electricity thereby allowing the active-matrix substrate in a state before being assembled into a display device to be subjected to an electric test. In the case of the shortcircuiting ring having a relatively high resistance, since the ring is usually grounded at a certain point of the ring to discharge static electricity, the resistance between the grounded point and a signal line significantly spaced apart therefrom is remarkably high. For example, if there are one hundred signal lines, the resistance between the grounded point and the line at the farthest location therefrom is 100 times as high as the resistance between the grounded point and the line at the nearest location therefrom. Accordingly, uniform discharge effect is impossible for all the lines. In addition, there is no disclosure about a specific range of the electric resistance required for the countermeasure against the problem of static electricity. If the resistance (hereinafter referred to as "resistance") of the shortcircuiting ring is set to a value stably allowing the discharge of static electricity for, for example, one hundred lines, the resistance between adjacent lines becomes low and, hence, an electric test such as for shortcircuiting between adjacent lines cannot be satisfactorily accomplished.
Japanese Unexamined Patent Publication No. 219662/1987 has disclosed an art of interconnecting gate lines, drain lines, and gate lines and drain lines through amorphous silicon doped with an impurity to discharge static electricity thereby avoiding insulation breakdown and allowing an electric test. Like the foregoing Japanese Unexamined Patent Publication No. 116117/1991, this art also causes a problem that the resistance between the grounded point and a line spaced apart therefrom is equal to the sum of respective resistances between lines lying therebetween of which resistances are connected in series, resulting in unsatisfactory discharge of static electricity. In addition, although the resistance of the amorphous silicon film doped with an impurity is not mentioned therein, such an amorphous silicon film typically has a sheet resistance of several mega-ohms (M.OMEGA.) or higher per unit area and, hence, it is difficult to obtain a satisfactory shortcircuiting effect for allowing static electricity discharge.
Japanese Unexamined Utility Model Publication No. 3827/1989 has disclosed in the specification thereof an art of connecting each gate line terminal and each drain line terminal to a common electrode line (shortcircuiting ring) through a high-resistance layer to avoid breakage of a component such as TFT due to static electricity and to enable an electric test, wherein an amorphous silicon film having a resistance of several M.OMEGA. is used as the high-resistance layer. However, the high-resistance Si film having a resistance of several mega-ohms (M.OMEGA.) does not enable satisfactory discharge of static electricity.
It is, therefore, an object of the present invention to provide an active-matrix substrate wherein a thin film resistor of 10 to 500 k.OMEGA. is provided intermediate between each of source and gate signal lines and a shortcircuiting ring of low resistance thereby assuredly avoiding a break of a component such as TFT due to static electricity while allowing the substrate in a state prior to being assembled into a display to be subjected to an electric test for the characteristics of each pixel component such as TFT and for a possible shortcircuit between source signal line and gate signal line, between source signal lines or between gate signal lines. Such an active-matrix substrate will contribute to improvements in production yield and product quality.
It is another object of the present invention to provide an active-matrix substrate having a thin film resistor formed of a silicide film which can be fabricated with fewer production steps.
Intensive study has been repeatedly made to obtain an active-matrix substrate which is prevented from troubles such as a break down of a signal line or TFT or a breakdown of an interlayer insulator film intermediate signal lines and which, in a state prior to being assembled into a display, is capable of undergoing a shortcircuit test for a possible shortcircuit between signal lines. As a result, there has been found the fact that by connecting the input terminal of each signal line to a shortcircuiting ring through a thin film resistor of 10 to 500 k.OMEGA., static electricity is discharged through the shortcircuiting ring while the thin film resistor causes leakage current to decrease when the substrate undergoes a test for the characteristics of each pixel component such as TFT. This means that discharging of static electricity stands together with the leakage test. More specifically, since the substrate is usually charged with static electricity of a high voltage in the range of about several hundred volts (V) to about several kilo-volts (kV) and a capacitance of about 0.1 to about 1 nF exists between the gate lines and the source lines in the TFT array, each TFT is protected from troubles such as shifting of its threshold voltage V.sub.th while a break due to discharge of static electricity between signal lines is prevented by decreasing the voltage of static electricity to 50 V or below in about 0.01 seconds. To decrease the voltage of static electricity to 50 V or below in 0.01 seconds, a time constant of 0.001 seconds is required and, hence, the resistance of an electric path from the input terminal of each signal line through the shortcircuiting ring to the grounded point is required to be set to 500 k.OMEGA. or lower.
On the other hand, when the active-matrix substrate undergoes the test in a state before assembled into a display device, a method is usually employed of measuring the amount of electric charge accumulated within a pixel. Since this method is implemented by applying a usual drive voltage to a gate signal line and a source signal line and measuring the current in the source signal line, the shortcircuiting ring becomes a shunt circuit. It has been found that by setting the shunted current in the shortcircuiting ring to 1/10 or lower of the current in a pixel, the test for characteristics of pixels such as TFT can be accurately achieved. Since the resistor connected to the shortcircuiting ring is in parallel with a resistor connected to the circuit, such as an integrating circuit, of the device for measuring the current in the pixel and the latter resistor typically has a resistance of not higher than 1 k.OMEGA., the former resistor is required to have a resistance ten times or more as high as the resistance of the latter resistor, i.e., 10 k.OMEGA. or higher so as to set the shunted current in the shortcircuiting ring to 1/10 or lower of the current in the pixel.
Thus, the provision of the thin film resistor of 10 to 500 k.OMEGA. intermediate between the input terminal of each signal line and the shortcircuiting ring allows the active-matrix substrate to be protected from troubles due to static electricity and to undergo an electric test for the characteristics of each pixel and for a possible shortcircuit between signal lines. A shortcircuit between signal lines can readily be detected by the aforementioned method of measuring the amount of electric charge at each pixel since the current abnormally increases due to the shortcircuit. In addition, the use of the thin film resistor of ITO or silicide makes it possible to easily obtain a resistance of about 10 to about 500 k.OMEGA. while allowing the thin resistor itself to be formed simultaneously with the electrodes or TFTs in the same fabrication process.